One man’s opinion.
A wise old Captain once told me, Upgrade at your first chance because you never know when the brass ring will swing away and possibly lock you out for what could be several years.
I upgraded just about as soon as I could (AA.) LGA 737. Took 17 years almost to the day. Luckily our junior upgrade bases are LGA and MIA, and since I live in the DC area I figured commuting to LGA reserve wouldn’t be too bad. Guess what, 10 other guys also got the upgrade along with me, all junior to me, so I waltzed right in to holding Long Call Reserve (12hr callout) which made it a breeze. Then, movement started happening so fast that I started holding a line only three months later.
I did it mainly for the huge 401k increase on the back end of the career, as well as the huge monthly pay raise (significant.) Lastly, life in the right seat was getting kinda stale after 17 years, lol. The 16% company input 401k gains after you upgrade are magnified many $$$ thousands over when you retire. Huge retirement gains make it worth it right there by itself.
Have not regretted the decision one bit. Highly recommend it as long as it doesn’t trash your QOL.
That said, if your airline has a decent wide body fleet and you’d be a senior WB FO making the same coin as a junior NB CA, I can see that being a smart decision too.